1. Field of the Invention
The present invention is directed to a method for feedback recognition in a hearing aid and is also directed to a hearing aid with feedback recognition. The invention can be utilized for all hearing aid embodiments and technologies, for example for behind-the-ear or for in-the-ear hearing aids that can be constructed in analog or digital circuitry or in mixed forms.
2. Description Of The Prior Art
There is the general problem in hearing aids of an undesired acoustic feedback between the audio transducer and the microphone. Such a feedback can cause whistling noises or other disturbances and thus considerably diminishes the utility of the hearing aid for the hearing aid wearer or can even reduce it to zero. Dependent on the properties of the hearing aid and the auditory situation, feedback can occur at different frequencies.
Most hearing aids currently on the market exhibit no particular devices for feedback recognition and feedback suppression. Feedback in such devices can only be avoided by a correspondingly low gain setting (as a preventative measure or as needed by the user). This measure, however, also reduces the utility of the hearing aid for users who require high amplification.
European Application 0 415 677 discloses a hearing aid having a negative feedback path. A filter connected in the feedback path models the property of the acoustic transmission path from the audio transducer to the microphone in order to compensate acoustically fed back signal parts. The quality of the feedback suppression achieved in this way, however, is highly dependent on the properties of the filter. Moreover, the hearing aid disclosed in European Application 0 415 677 is technologically complicated.
An object of the present invention is to provide a method and a hearing aid which reliably recognize feedback occurring in the hearing aid, so that suitable measures for feedback suppression can be undertaken. A further object is to provide such a method and hearing aid wherein the feedback recognition occurs automatically and dependably in a broad frequency range and is inaudible for the hearing aid wearer.
The above object is inventively achieved in a method and a hearing aid wherein feedback is recognized by optionally attenuating a frequency band in which feedback could occur in the signal transmission path between the microphone and the audio transducer. A signal level of the signal transmitted on the signal transmission path is determined without or with the added attenuation. When feedback arises in the monitored frequency band, the signal level is reduced more greatly by the attenuation than would be expected without feedback. The fed back signal components multiply pass through the attenuation unit, so that the feedback is highly diminished or is completely eliminated. As a result, feedback can be dependably recognized and suitable counter-measures can be undertaken.
Preferably, the feedback recognition is implemented multiply or continuously in different frequency bands. The entire frequency range in which feedback can occur (for example, the entire transmission range of the hearing aid) thus can be constantly monitored. In alternative embodiments of the hearing aid, only a few frequency ranges are monitored; only a single one in the extreme case. This can be meaningful when only specific feedback frequencies are anticipated in specific hearing aid structures or when especially simple and low cost bandpass and/or band stop filters are utilized.
Since the attenuation is inventively connected into the ordinary signal transmission path of the hearing aid, it is desirable to optimally reduce the audible influence of the attenuation. In preferred embodiments, this occurs by attenuation in a relatively narrow frequency band and/or by attenuation which is only of a brief-duration attenuation and/or by a relatively slight degree of attenuation. As a result of one or more of these measures, feedback recognition that is imperceptible to the hearing aid user can be achieved.
The selection of a narrow attenuation frequency band (attenuation characteristic with high edge steepness) is especially meaningful whenxe2x80x94as already mentionedxe2x80x94the feedback suppression is repeatedly implemented with different frequency bands. The width of the attenuated frequency band (distance between the corner frequencies) can, for example, amount to 100 Hz through 2 kHz.
In other embodiments, the duration of the attenuated operation amounts to only respectively 0.5 ms through 50 ms, preferably 5 ms. With, for example, a frequency from 10 Hz through 1 kHz, preferably 100 Hz, a switch can thus be undertaken between attenuated and non-attenuated operation. The time ratio between these two operating modes can amount to 1:1 or, for example, can provide a longer unattenuated operation. When a narrow frequency band is attenuated for only short time intervals, the degree of attenuation can be relatively high and, for example, amount to 10 dB. A high recognition dependability is thereby achieved and disturbance to the hearing aid user is nonetheless avoided.
In order to recognize a feedback situation, signal intensities are determined with and without attenuation. The signal intensities preferably are determined only in that frequency band that is also subject to the attenuation. As a result, an especially high recognition dependability and insensitivity to noise is achieved. In alternative embodiments of the hearing aid, however, signal intensities of broader frequency bands can be identified or, in the extreme case, those of the entire transmission spectrum.
Preferably the attenuation of the signal intensity due to the attenuation connected into the signal transmission path is used as a criterion for the feedback recognition. For example, it can be determined whether the signal attenuation exceeds a predetermined limit value. This limit value, for instance, can be two or three times as high as the attenuation which is anticipated given non-fed back hearing aid operation.
In preferred embodiments, the attenuation frequency band is variable in steps or continuously. For monitoring a broad frequency range, this can be steadily stepped or xe2x80x9ccriticalxe2x80x9d frequencies can be more frequently selected. In preferred developments, the frequency band for the monitoring is selected on the basis of a pre-examination of the signal transmitted by the hearing aid. Thus, for example, the attenuation frequency band can be set to frequencies of an especially loud signal part. Alternatively or additionally, an oscillation detector can be employed that reacts to (sine-shaped) feedback tones. Such an oscillation detector can, for example, determine the time intervals between zero-axis crossings in the hearing aid signal and can evaluate them for setting the attenuation frequency band.
In embodiments of the invention, feedback that has occurred is not only recognized but is also largely or completely suppressed. The attenuation of the hearing aid signal in accordance with the invention can already serve this purpose when it is strong enough in order to cause the feedback to subside. Alternatively or additionally, other devices can be provided for feedback suppression, for example an amplifier with variable gain and/or a further band stop filter and/or a phase shifter or which acts on the hearing aid signal.